Synchronisation System Designs Chosen for SKA telescopes

Left: Synchronisation distribution system designed by ICRAR selected for SKA-mid dishes in South Africa. Right: Synchronisation distribution system designed by Tsinghua University selected for SKA-low antennas in Australia. Credit: ICRAR / Tsinghua University

SKA Global Headquarters, Jodrell Bank, UK, Wednesday 11 October – On Monday the Board of the SKA’s international Signal and Data Transport (SaDT) consortium selected the synchronisation distribution system designs to be used for both SKA telescopes, endorsing the decision of a panel of leading experts in the field of time synchronisation.

While optical fibres are incredibly stable and suited to transport data, mechanical stresses and thermal changes do affect the fibre, degrading the stability of the transmitted signals over long distances.

The long distances between the SKA antennas means radio waves from the sky reach each antenna at different times. With eventually thousands of antennas spread over continental scales and therefore thousands of kilometres of fibre, one of the most complex technical challenges for the SKA to function properly is to make sure the signals from the antennas are aligned with extreme precision to be successfully combined by the SKA’s supercomputers.

“Given the scale of the SKA, this is an engineering problem that hadn’t really been faced before by any astronomical observatory” said André Van Es, the SaDT Engineering Project Manager supervising the consortium’s work for SKA Organisation (SKAO).

To achieve this level of precision or “coherence” across the array, the SKA requires a synchronisation distribution system that supresses these fibre fluctuations in real time.

“The performance required is for less than 2% coherence loss. Bearing in mind a 1% loss is equivalent to losing two dishes or antenna stations, it’s crucial that we get this right for the telescopes to be effective” explained SKAO timing domain specialist Rodrigo Olguin.

The pulses sent by the synchronisation distribution system travel to each antenna using the optical fibre network also used for transporting astronomical data to the SKA’s central computer. The system then takes into account the mechanical stresses and thermal changes in the fibre and corrects the timing difference to make sure all signals coming from the antennas are digitised synchronously.

“This decision based on the SKA’s requirements combines both cost-effectiveness and reliability of the designs, resulting in an optimal two-system solution for the telescopes” explained André Van Es.

A Sub-Rack enclosure used to hold 16 of the 197 Transmitter Modules for the SKA-mid phase synchronisation system. One prototype Transmitter Module is shown partly extended from the front of the enclosure, revealing details of the system’s critical fibre-optic components. Credit: ICRAR.

“Our SKA frequency synchronisation system continuously measures changes in the fibre link and applies corrections in real-time with fluctuations of no more than five parts in one-hundred trillion over a 1-second period”, said lead designer, Dr Sascha Schediwy from ICRAR and The University of Western Australia (UWA). “A clock relying on a signal of that stability would only gain or lose a second after 600,000 years.”

Main card and Transmitting card of a module from the frequency dissemination system designed by Tsinghua University for the SKA-low telescope. Credit: Tsinghua University

Dr. Bo Wang of Tsinghua University explains “Our system employs a frequency dissemination and synchronisation method that features phase-noise compensation performed at the client site. One central transmitting module can thus be linked to multiple client sites, and future expansion to additional receiving sites can be realised without disrupting the structure of the central transmitting station.”

The very accurate timing and synchronisation systems will enable the SKA to contribute to many fields from mapping the distribution of hydrogen in the Universe over time to studying pulsars and detecting gravitational waves on a galactic scale, making it complementary to the LIGO & VIRGO gravitational wave observatories.

The synchronisation system designs chosen were developed as part of the SaDT Consortium led by Prof. Keith Grainge of the University of Manchester, UK and which includes institutes from eight countries, including the University of Western Australia and Tsinghua University from Beijing, China. SaDT is responsible for the transmission of SKA data and the provision of timing, across two telescope-wide networks. Read more about SaDT’s work: http://www.skatelescope.org/sadt/

About the SKA

The Square Kilometre Array (SKA) project is an international effort to build the world’s largest radio telescope, led by the SKA Organisation based at the Jodrell Bank Observatory near Manchester. The SKA will conduct transformational science to improve our understanding of the Universe and the laws of fundamental physics, monitoring the sky in unprecedented detail and mapping it hundreds of times faster than any current facility.

The SKA is not a single telescope, but a collection of telescopes or instruments, called an array, to be spread over long distances. The SKA is to be constructed in two phases: Phase 1 (called SKA1) in South Africa and Australia; Phase 2 (called SKA2) expanding into other African countries, with the component in Australia also being expanded.

Already supported by 10 member countries – Australia, Canada, China, India, Italy, New Zealand, South Africa, Sweden, The Netherlands and the United Kingdom – the SKA Organisation has brought together some of the world’s finest scientists, engineers and policy makers and more than 100 companies and research institutions across 20 countries in the design and development of the telescope. Construction of the SKA is set to start in 2019, with early science observations in the early 2020s.